As a form of training, very suitable (3)
I believe the answer is:
apt
'suitable' is the definition.
(apt means appropriate or fitting)
'as a form of training very' is the wordplay.
I cannot quite see how this works, but
'a' is present in the answer.
'training' could be 'pt' (Physical Training) and 'pt' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for apt that I've seen before include "Fitting - inclined" , "Having a tendency" , "Fit, suitable" , "To the point" , "Train" .)